The present invention relates to memories and more particularly to hierarchial memories.
Hierarchial memories using monolithic storage elements at the higher levels have resulted in faster and more economical memory systems. However, such memories are not without drawbacks. In particular, hierarchial systems have significant communication problems between the levels of the hierarchy and the volatile monolithic memories in the higher levels of the hierarchy have data integrity problems resulting from their reliability and their susceptibility to power supply failures and fluctuations. One solution to the integrity problem would be to have two storage units at each level, each with a duplicate of the data at the level so that if a failure occurred in one store, the data could be reproduced from the other. However, while this improves reliability it does nothing to alleviate the communication problem between the levels and it doubles the number of storage elements needed to form any given level of the memory within increasing the capacity of the memory. Furthermore, in case of a failure requiring or forcing a shut down of the memory the transfer of the duplicated data from the levels of the memory containing the volatile memories to levels containing non-volatile memories in the process of bringing the storage system down would require much more time than is desirable and in fact may be available before the failure results in the destruction of data.
In co-pending appliction Ser. No. 486,043, filed July 5, 1974, now issued into U.S. Pat. No. 4,020,466 data integrity is obtained using new copy back techniques and the communication problem has been simplified by using new copy back algorithms in combination with the new copy back techniques. The new hierarchial memory system has two memory units on each level. One of the units contains all the data at that level of the memory. The other unit contains only the changes that have been made in that data either by addition or modification. While the first or data storage unit is interfaced with the next higher level in the hierarchial memory system or with the processing units for the data processing system, the second or copy back storage unit can transfer the changes made in the data into the next lower level in the memory hierarchy system. if the copyback store is free and the main memory unit in the next lower level is not involved in transferring of data up the line. By duplicating only the changed data and copying back the changes to the data at lower levels when the opportunity presents itself, the amount of data that must be moved at the occurrence of a failure is minimized thereby reducing the time required to move the data on failure. Furthermore, the data store and the copy back data store in each level are on two different power systems so that if either power system goes out a complete record of the data is maintained. However, as described in the above-identified patent application, the copy back data store at any given level is as large as the data store at that level. In this respect the copy back technique of the above described application has the same effect on the memory as duplicating all the data at each level. That is, it doubles the number of storage elements on each level of the hierarchy without a resultant increase in the capacity of the memory.